The present invention relates to a process for the production of diphenyl ether compounds which are useful as herbicides. In particular, it relates to a process for obtaining herbicidal diphenyl ether products on an industrial scale.
The problems associated with producing diphenyl ether herbicides on an industrial scale are discussed in WO 97/10200 and WO 97/10199 in which there are disclosed processes for making compounds of formula 
wherein R1 is hydrogen or C1-C6 alkyl, C2-C6 alkenyl or C2-C6 alkynyl (any of which may optionally be substituted with one or more substituents selected from halogen and OH) or COOH, COH, COOR4, COR6, CONR4R5 or CONHSO2R4; R4 and R5 are each independently hydrogen or C1-C4 alkyl optionally substituted with one or more halogen atoms; R6 is a halogen atom or a group R4; R2 is hydrogen or halo; R3 is C1-C4 alkyl, C2-C4 alkenyl or C2-C4 alkynyl, any of which may optionally be substituted with one or more halogen atoms, or halo or, where appropriate, a salt thereof. More especially the disclosures deal with the commercially known herbicides 5-(2-chloro-xcex1,xcex1,xcex1-trifluoro-4-tolyloxy)-2-nitrobenzoic acid (acifluorfen) and 5-(2-chloro-xcex1,xcex1,xcex1-trifluoro-4-tolyloxy)-N-methanesulphonyl-2-nitrobenzamide (fomesafen).
A preferred method of making fomesafen on an industrial scale is via acifluorfen which is made from 5-(2-chloro-xcex1,xcex1,xcex1-trifluoro-4-tolyloxy)-benzoic acid (CTTBA). This compound is produced by oxidation of the corresponding tolyl compound which is in turn obtained from the condensation of 3-hydroxybenzoic acid and 3,4-dichlorobenzotrifluoride.
Because the final product (fomesafen) is required in sufficient purity to meet strict product registration standards the process normally also involves one or more purification procedures.
The synthesis of acifluorfen and its conversion to fomesafen has been the subject of intensive research in an effort to improve one or more of the process steps. Thus for example in WO97/10200 there is disclosed a purification method for acifluorfen or fomesafen, while a set of improved nitration conditions is disclosed in WO97/10199. Another set of possible nitration conditions is disclosed in WO 98/19978. However no processes are known which can perform most or all of the above reactions using a single solvent. This is undoubtedly because the range of reaction conditions is very demanding and it is not at all apparent if a process using a single common solvent is possible.
The applicants have now devised a single common solvent process in which acifluorfen is converted to purified fomesafen. There is therefore provided a process for producing fomesafen from acifluorfen comprising the steps of
converting acifluorfen to its acid chloride 
coupling the acid chloride so formed with methanesulphonamide (MSAM) to form crude fomesafen and 
purifying the crude fomesafen characterised in that each of the steps is carried out in a single common solvent.
The product of each step may be isolated at the end of the step or, more preferably, the steps may be telescoped together so that there is no isolation until the purified fomesafen end product is obtained.
Suitable solvents are haloalkanes (such as 1,2-dichloroethane or tetrachloroethylene), halobenzenes (such as fluorobenzene, chlorobenzene and dichlorobenzenes), alkoxybenzenes (such as anisole or phenetole), haloalkylbenzenes (such as benzotrifluoride), and esters (such as ethyl acetate or butyl acetate). Preferred solvents are chloroalkanes especially 1,2-dichloroethane (or ethylene dichloride or EDC).
In a further aspect of the invention the acifluorfen is formed by nitration of 
CTTBA in the same solvent used to convert acifluorfen to fomesafen, the solvent being a chloroalkane, especially EDC.
In yet a further aspect of the invention the CTTBA is formed by the oxidation of the corresponding toluene and the CTTBA is extracted from the reaction mass using the same solvent that is used to convert acifluorfen to fomesafen, the solvent being a chloroalkane, especially EDC.
The CTTBA is suitably generated by the oxidation of the corresponding toluene using oxygen together with a catalyst (such as a cobalt or vanadium salt), at a temperature of 70xc2x0 C. to 150xc2x0 C.
The product of the extraction step and/or the nitration step may be isolated at the end of the step or, more preferably, the steps may be telescoped together so that there is no isolation of product at the end of each of the extraction and nitration stages.
In one suitable method for the nitration reaction, the nitrating agent may be nitric acid or a mixture of nitric and sulphuric acids although other types of nitrating agent may also be used. It is also advantageous to conduct the reaction in the presence of acetic anhydride and, in this case, it is preferred that the molar ratio of acetic anhydride to CTTBA is from about 1:1 to 3:1. The reaction temperature may be from about xe2x88x9215xc2x0 to 15xc2x0 C., more usually from about xe2x88x9210xc2x0 to 10xc2x0 C. It is advantageous to add the nitrating agent, over a period of time from 5 to 15 hours, or, more preferably, 6 to 12 hours. Most preferably the reaction is carried out using a mixture of nitric and sulphuric acids at 0-5xc2x0 C. The resultant product solution is water washed, to remove mineral acid and acetic acid then topped to remove residual water.
The conversion of acifluorfen to the acid chloride may be carried out by conventional methods, for example as set out in EP-A-0003416. It is preferred to perform the reaction with a suitable chlorinating agent such as thionyl chloride or phosgene in the presence of a catalyst such as triethylamine or dimethylformamide at temperature of 60 to 80xc2x0 C., preferably 70xc2x0 C. The acid gases (SO2 and HCl) may be removed, together with excess of the chlorinating agent by addition and distillative removal of further organic solvent.
The acid chloride may then be reacted with methane sulphonamide to give fomesafen. This step may suitably be carried out by conventional methods, for example as set out in EP-A-0003416. In a preferred process the acid chloride is coupled with MSAM using an excess of base such as potassium carbonate at 60 to 80xc2x0 C., preferably at 80xc2x0 C. The inorganic by-products and excess MSAM are removed by washing with water. The final product solution is topped to remove water. The organic solvent level, in the final product solution, is adjusted by topping off excess or by further additions and the product is then isolated by cooling to xe2x88x9210 to 30xc2x0 C. preferably 0xc2x0 C.
A particular advantage in the use of EDC in the coupling reaction with MSAM is that the boiling point of the solvent is the upper temperature limit for process operation and thermal decomposition of the acid chloride during the reaction is avoided.
The product may then be purified using a chloroalkane solvent. The purification may suitably involve filtration and washing with cold solvent or further recrystallisation from the solvent.
Using the process invention, it is possible to obtain fomesafen of desired quality in good yield enabling the industrial operation of the process with a single solvent.
The invention will now be further illustrated with reference to the following examples.